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\(P=\left(\frac{\left(\sqrt{x}-\sqrt{y}\right)\left(1-\sqrt{xy}\right)+\left(\sqrt{x}+\sqrt{y}\right)\left(1+\sqrt{xy}\right)}{1-xy}\right):\left(\frac{x+y+2xy+1-xy}{1-xy}\right)\)
\(=\left(\frac{2\sqrt{x}+2y\sqrt{x}}{1-xy}\right):\left(\frac{\left(x+1\right)\left(y+1\right)}{1-xy}\right)\)
\(=\frac{2\sqrt{x}\left(y+1\right)}{\left(1-xy\right)}.\frac{\left(1-xy\right)}{\left(x+1\right)\left(y+1\right)}=\frac{2\sqrt{x}}{x+1}\)
\(x=\frac{2}{2+\sqrt{3}}=\frac{2\left(2-\sqrt{3}\right)}{4-3}=4-2\sqrt{3}=\left(\sqrt{3}-1\right)^2\Rightarrow\sqrt{x}=\sqrt{3}-1\)
\(\Rightarrow P=\frac{2\left(\sqrt{3}-1\right)}{5-2\sqrt{3}}=\frac{2+6\sqrt{3}}{13}\)
Ta có \(1-P=1-\frac{2\sqrt{x}}{x+1}=\frac{x-2\sqrt{x}+1}{x+1}=\frac{\left(\sqrt{x}-1\right)^2}{x+1}\ge0\) \(\forall x\ge0\)
\(\Rightarrow1-P\ge0\Rightarrow P\le1\)
a)Áp dụng BĐT AM-GM ta có
\(\frac{ab\sqrt{ab}}{a+b}\le\frac{ab\sqrt{ab}}{2\sqrt{ab}}=\frac{ab}{2}\)
Tương tự cho 2 BĐT còn lại cũng có:
\(\frac{bc\sqrt{bc}}{b+c}\le\frac{bc}{2};\frac{ac\sqrt{ac}}{a+c}\le\frac{ac}{2}\)
Cộng theo vế 3 BĐT trên ta có:
\(VT=Σ\frac{ab\sqrt{ab}}{a+b}\le\frac{ab+bc+ca}{2}=VP\)
Khi \(a=b=c\)
b)Áp dụng tiếp AM-GM:
\(b\sqrt{a-1}\le\frac{b\left(a-1+1\right)}{2}=\frac{ab}{2}\)
\(a\sqrt{b-1}\le\frac{a\left(b-1+1\right)}{2}=\frac{ab}{2}\)
Cộng theo vế 2 BĐT trên ta có:
\(VT=b\sqrt{a-1}+a\sqrt{b-1}\le ab=VP\)
Khi \(a=b=1\)
\(S=\frac{\sqrt{a-2}}{a}+\frac{\sqrt{b-6}}{b}+\frac{\sqrt{c-12}}{c}=\frac{\sqrt{2\left(a-2\right)}}{\sqrt{2}a}+\frac{\sqrt{6\left(b-6\right)}}{\sqrt{6}b}+\frac{\sqrt{12\left(c-12\right)}}{\sqrt{12}c}\)
\(\le\frac{\frac{2+a-2}{2}}{\sqrt{2}a}+\frac{\frac{6+b-6}{2}}{\sqrt{6}b}+\frac{\frac{12+c-12}{2}}{\sqrt{12}c}=\frac{a}{2\sqrt{2}a}+\frac{b}{2\sqrt{6}b}+\frac{c}{2\sqrt{12c}}\)(AM-GM)
\(=\frac{1}{2\sqrt{2}}+\frac{1}{2\sqrt{6}}+\frac{1}{2\sqrt{12}}\)
Dấu "=" xảy ra \(\Leftrightarrow a=4;b=12;c=24\)
Áp dụng bđt cosi ta được \(a+b+c\ge2\sqrt{a\left(b+c\right)}\Leftrightarrow\frac{a}{\sqrt{a\left(b+c\right)}}\ge\frac{2a}{a+b+c}\Leftrightarrow\sqrt{\frac{a}{b+c}}\ge\frac{2a}{a+b+c}\)
tương tự \(\Rightarrow\sqrt{\frac{a}{b+c}}+\sqrt{\frac{b}{a+c}}+\sqrt{\frac{c}{a+b}}\ge\frac{2\left(a+b+c\right)}{a+b+c}=2\)
dấu = xảy ra khi a=b+c ; b=c+a ; c=a+b => a=b=c=0 (vo lí ) => k xảy ra dấu ==> dpcm
\(\sqrt{a^2+\dfrac{1}{b+c}}=\dfrac{2}{\sqrt{17}}\sqrt{\left(4+\dfrac{1}{4}\right)\left(a^2+\dfrac{1}{b+c}\right)}\ge\dfrac{2}{\sqrt{17}}\left(2a+\dfrac{1}{2\sqrt{b+c}}\right)\)
\(\Rightarrow A\ge\dfrac{1}{\sqrt{17}}\left(4a+4b+4c+\dfrac{1}{\sqrt{a+b}}+\dfrac{1}{\sqrt{b+c}}+\dfrac{1}{\sqrt{c+a}}\right)\)
\(\Rightarrow A\ge\dfrac{1}{\sqrt{17}}\left(4a+4b+4c+\dfrac{9}{\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a}}\right)\)
Mặt khác:
\(\sqrt{a+b}+\sqrt{b+c}+\sqrt{c+a}\le\sqrt{3\left(a+b+b+c+c+a\right)}=\sqrt{6\left(a+b+c\right)}\)
\(\Rightarrow A\ge\dfrac{1}{\sqrt{17}}\left(4a+4b+4c+\dfrac{9}{\sqrt{6\left(a+b+c\right)}}\right)\)
\(\Rightarrow A\ge\dfrac{1}{\sqrt{17}}\left(\dfrac{31}{8}\left(a+b+c\right)+\dfrac{a+b+c}{8}+\dfrac{9}{2\sqrt{6\left(a+b+c\right)}}+\dfrac{9}{2\sqrt{6\left(a+b+c\right)}}\right)\)
\(\Rightarrow A\ge\dfrac{1}{\sqrt{17}}\left(\dfrac{31}{8}.6+3\sqrt[3]{\dfrac{81\left(a+b+c\right)}{32.6.\left(a+b+c\right)}}\right)=\dfrac{3\sqrt{17}}{2}\)
Dấu "=" xảy ra khi \(a=b=c=2\)